HPCC-10227 - Use key cache in indexread/count

Particularly critical when performing index counts in child
queries. Without cachine the parts are continually reopened at
a rapid rate which can cause network issues.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>
Merge pull request #5019 from jakesmith/hpcc-10227

HPCC-10227 - Use key cache in indexread/count

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>